Coordinate measuring machines are typically arranged for capturing point-by-point measurements along the surfaces of test objects using probes whose displacement with respect to the surfaces is measured within a common coordinate system. The measures typically include measurements of the relative motions of the probes with respect to the test objects along or about motion axes of the machines as well as measurements of the relative displacements of the probes with respect to the surfaces of the test objects.
The probes can take various forms including physical probes that contact the surfaces of test objects and optical probes, sometimes referred to as optical pens, that reflect focused light from the surfaces of the test objects. The reflected light can be analyzed as a measure of distance.
For example, U.S. Pat. No. 8,817,240 of Jones et al. describes an optical probe that exploits chromatic dispersion through confocal optics for focusing different wavelengths at different focal lengths and measuring distance as a function of the wavelength that is reflected through the confocal aperture. For varying the range or direction of measurement, the '240 patent divides the optical pen into a base member that includes the confocal aperture and an interchangeable element that includes chromatically dispersive optics. A repeatable fast exchange mount allows the interchangeable element to be replaced by another interchangeable element having different chromatically dispersive optics or other features for varying the range or direction of measurement.
Another type of optical probe for taking distance measurements with a partially coherent interferometer is described in U.S. Pat. No. 7,791,731 of Kay, who is also the named inventor of the subject application. A spatially coherent source beam composed of multiple wavelengths, i.e., a low temporally coherent beam, is divided by a beamsplitter into an object beam that is reflected from the test object and a reference beam that is reflected from a reference reflector. Reflected light from both the test object and the reference reflector is recombined at a beamsplitter into a measurement beam and refocused within a detector such as a spectrometer, which records interference intensities of the different spectral components of the returning measurement beam. The rate of change in interference intensities over the captured spectrum is equated to a measure of distance. Exchanging probe parts containing entire interferometers is particularly problematic because of issues of bulk and cost, and exchanging parts of interferometers is particularly problematic because any differences can change the measurement results.